We can use a factor tree. You'll find it's factors then the factors of the composite numbers which are also factors of that number. Lemme give you an example..
Find the GCF of 50 and 39. (Click image below) Since, 50 and 39 has no common prime factors, their GCF is 1. If you found that numbers have common prime factors, select them and multiply them to get the numbers' GCF.
Prime factorization of 50 and 39:
50 = 2 × 5² (same as 2 × 5 × 5)
39 = 3 × 13
I hope it help. ☺
